The Technologies Behind the Internet

Transcription

1 The Technologies Behind the Internet Lecture 3 Oct. 6, 2016 Lincoln Towers University Thursdays 7:30-9 pm, 150 WEA Community Room Instructor: Stephen Weinstein s.weinstein@ieee.org, (646) Lecture notes posting site: projectopenlincolntowers.org/lincolntowersuniversity

2 Today: Internet architecture & technologies -Protocol stacks. -Datagrams, packet switches, and routing protocols -Internet architecture (ISPs, NAPs, DNS), translating a web address to an IP address. -Important protocols: IP, TCP, UDP, OSPF, DHCP

3 Lecture 4 (April 28): Internet applications -The original application level protocols: ftp, smtp, telnet -The World Wide Web: History, browsers, and web pages -Audio and video streaming, VoIP (e.g., Skype). -Virtual private networks (secure tunnels). -Cloud computing. -Security attacks (e.g., denial of service) and defenses. -The Internet of Things.

4 Lecture 3: Internet architecture and technologies The Internet provides a best effort (datagram) packet transfer service across diverse physical networks; a reliable connection-oriented service on top of that; and support for name-to-internet-address translation. These will all be explained. Physical channels Name-address services

5 Protocol and protocol stack

6 Protocol definition (from first lecture) A formal description of the format and rules for a message exchange. Several layers of protocols are usually needed to completely specify an information exchange across one or more networks.

7 Protocol stack: A way to reduce complexity -Modularizes functions. Permits rewriting the function of one protocol level without having to rewrite all the other levels ( mix and match ). -Facilitates interoperability between diverse equipment from different manufacturers (open systems interconnection). Physical network link

8 International Standards Organization (ISO) reference model for Open Systems Interconnection (OSI) Application: Application-dependent services & procedures Presentation: Data formats, representations, and displays Session: Control of dialog between processes Transport: End-to-end (system, application) info transfer Network: Routing, message or packet structure Link or MAC: Ordered data flow on links, access arbitration Physical: Electrical, Mechanical & functional interfaces

9 Internet defined by protocol stack RTP, RTSP Applications Internet HTML Session level HTTP Transport level Network level TCP, UDP IP, Routing protocols, DiffServ Independent of all beneath this line Medium Access Control (MAC) or Link layer Physical networking (Wired and wireless signaling, data framing, modulation, etc.)

10 Each protocol layers offers a service to the next higher layer, and interacts with its cousin, at the same level, across the network Application requests transport service Interfaces Information unit transfer Application Transport Network requests media access, framing, & line signaling Physical network Link, Phys Transport data package transfer requests internetworking service Packet transfer Transport Network Link, Phys physical interface

11 A data unit at a given layer is encapsulated in a data unit of the next lower layer Transport data unit Network data unit (packet) This is like packing a gift in a box addressed to the recipient (transport package), which in turn is packed into the trunk of a car (network vehicle, a packet).

12 Packet switches (routers) and routing protocols. Packet switching operates at level 3 (network) level of the protocol stack.

13 Packet: A data package conveying (in its payload), digital information representing part or all of a message. A series of packets in a transmission channel may carry parts of messages from different sources.

14 As discussed in Lecture 1, packet transmission is desirable because: Resilience: Ability to reroute packets if a link or node goes down. Original path Recovery path

15 Burst traffic: Ability to convey brief data bursts (like a keyboard entry) without the delay and complexity of setting up new switched lines. Small packet transmitted quickly, without circuit setup delay Flexibility: Ability to mix different kinds of traffic (computer bursts, voice, video) at different data rates.

16 WHAT IS THE DIFFERENCE BETWEEN CIRCUIT SWITCHING AND PACKET SWITCHING? Telephone circuit switch: Connects physical channels to complete a reserved communication path that is kept up for the duration of the call. Prior reservation made by call setup signaling. Simplest illustration of a telephone circuit switch: Call setup signaling Connections state Voice lines in Crossbar circuit switch Voice lines out [Actual switches are more sophisticated, assigning slots in a time-division frame.]

17 Internet packet switch: Transfers a packet from an incoming line to an outgoing line thought to be the best next hop to the destination, with no prior reservation. This is called a datagram service. Packets Lines in Routing table Packet switch Lines out Best next hop If the buffer for the desired output line is already full, the packet is dropped! Priority services are possible, e.g., high-priority traffic is sent to the head of the line.

18 PACKET SWITCH ( router ) in more detail Arriving TCP or UDP data unit encapsulated in layer 3 IP packet encapsulated in MAC layer 2 data unit Input ports Routing table Routing algorithm Link state information 1 card 1 N Destination address Line Line card Next hop (output port) selection Forwarder Scheduler Buffer(s) Packet transferred to designated output buffer Scheduler Buffer(s) Line card Output ports Line card M

19 Your wireless router contains a router plus a WiFi transmitter-receiver Wireless laptop Local-area network side Antenna Wi Fi transmitter & receiver Router Wide-area network side Ethernet to cable or optical modem Additional Ethernet outputs to computers, printers, etc.

20 Line card The router supports a variety of different line cards corresponding to different attached networks. Cisco Port OC-3 ATM Line Card Optical core network Line card Line card Passive optical network ATMoptical network Line card Line card Ethernet

21 Routing Protocols The question they address: What is the best next hop (part of a best route) to the destination? Destination Source Shortest number of hops? Least traffic? Highest capacity links?

22 Routing is decentralized Complexity is handled by decentralizing routing functions into autonomous systems that handle their own internal routing functions, and internetworking between autonomous systems. Autonomous System 1 Autonomous System 2 Autonomous System 3 An autonomous system is a set of routers and networks controlled by a particular organization or administrative entity. It uses interior routing protocols.

23 Interior routing protocols 1. Routing Information Protocol (RIP) Minimizes the number of hops to destination or to exit router from this autonomous system. Destination Source Routing table information exchanged among routers.

24 Interior routing protocols 2. Open Shortest Path First (OSPF) Allows routes to be selected dynamically based on the current state of the network, not just a static picture of how routers are connected. Link state database (info exchanged between routers) Destination Source $1 $4 $4 $5 $6 $3 $3 $3 $2 $2 $4 $1 $1 Green: 9 $1 Link state databases store costs of different links, and OSPF chooses a low cost route. Cost may be in delay, rate limits, etc., not dollars.

25 Exterior gateway routing protocols (between autonomous systems) Border Gateway Protocol (BGP) Path attributes how to reach other AS s en route to desired address Autonomous System 2 Autonomous System 1 Autonomous System 3 Exchanges ( advertisements ) of path attributes

26 Internet Architecture: ISPs, Network Access Points (NAPs), and the Domain Name System (DNS)

27 ISP Internet Service Provider, an operator of networks carrying Internet traffic and interconnection devices, and a vendor of Internet access services. Higher-level (backbone) ISPs charge lower-level ones for access, until local ISPs who charge users. NAP Network Access Point, connects national ISPs together. About a dozen in U.S., run by large carriers such as AT&T. MAE Metropolitan Area Exchange, interconnects regional ISPs.

28 High-Level Internet Architecture $ $ $ Ref:

29 Domain Name System (DNS) Internet domain: a realm of administrative autonomy, authority, or control identified by a domain name that follows the rules of the Domain Name System administered by IANA (Internet Assigned Numbers Authority). Top-level domains* (the last part of a web or address) are managed in IANA s DNS root zone. There are hundreds including.com,.org,.gov and all country codes such as.it and.cn. Domains corresponding to computers or applications are designated by a combination of one or more lower-level domains and a top-level domain. Domain name example: projectopenlincolntowers.org which is also a Universal Resource Locator (URL) for access to a World Wide Web site. *

30 A URL (universal resource locator) address is not an IP address, but it corresponds to an IP address, and the translation between them is done by DNS. Example: projectopenlincolntowers.org = ISP DNS resolver projectopenlincolntowers.org Internet Note: IP address is transmitted as a binary string, not decimal digits as shown here, but this decimal representation is the usual way of expressing it to humans.

31 How DNS produces the desired IP address from scratch: a hierarchical series of queries and responses In order to be processed by a router, a packet must have a numerical IP address. Sending to a web or address requires first the translation of domain name to IP address by a domain name server. 1. Enter URL in address field of a browser and hit go.

32 2. Operating system of computer sends a query to your ISP s DNS resolver [or another specified DNS service such as Google DNS ( )]. URL ISP DNS resolver Internet

33 3. The ISP s DNS resolver sends a request to one of the root DNS servers. The root server doesn t know the IP address for Project Open, so it returns the locations of several.org servers who should know. Internet ISP Root server

34 4. The ISP s DNS resolver then consults one of the.org servers, which returns the locations of servers that know about projectopenlincolntowers.org. Internet ISP Reply.Query.org DNS server

35 5. The ISP s DNS resolver, finally, consults a DNS server that knows all about projectopenlincolntowers.org. This server returns the IP address to the ISP who passes it back to the originating computer, which uses it to send a packet Query to Project Open ISP Query Low-level DNS server Internet Note: This is a worst case scenario in terms of the length of the resolution process. If the user has recently accessed URLs of the same domain, or other users relying on the same DNS resolver have done such requests, there will be no DNS resolution required, or it will be limited to the query on the local DNS resolver.

36 The Main Internet Protocols

37 Internet Protocol (IP) -Defines the format and transmission of a datagram ( best effort service without capacity reservation) through interconnected networks of routers and links. IP is a layer 3 (network) protocol. -Exists in IPv4 and IPv6 versions, differing principally in the size of the address field (how many devices can have their own IP address). -Invented earlier by Cerf and Kahn, but first standardized in RFC 791 (edited by Jon Postel). RFC: 791 INTERNET PROTOCOL DARPA INTERNET PROGRAM PROTOCOL SPECIFICATION September 1981 prepared for Defense Advanced Research Projects Agency Information Processing Techniques Office 1400 Wilson Boulevard Arlington, Virginia 22209

38 IPv4 datagram format 1 Bits Version Header Type of service Total datagram length length (in 32-bit words 33 Flag (fragment or not) 16-bit identifier Fragmentation offset 65 Time to live (Max. remaining hops) 97 Protocol above (e.g., TCP) Source IP address Header checksum Destination IP address Options (if any) (timestamp, route taken, ) 160 Data (usually a TCP or UDP data unit)

39 Packet fragmentation Different links in an internetwork may have different maximum packet sizes. A packet entering a link whose maximum packet size is less than the packet length, will be fragmented into two or more smaller packets. The fragmentation offset of one of those smaller packets tells you where it goes when the original packet is reassembled at the end. Offset1 = 0 Fragment 1 Offset2 Fragment 2 Offset3 Fragment 3

40 IPv4 addresses -Addresses are 32 bits long, shown in four sections separated by dots. Example for Project Open: (binary) or (decimal) -IP addresses belong to classes (next slide) corresponding to networks of different sizes. The first part of an address designates a network, and the rest a host (end device, like a laptop) on that network. Network ID Host ID Note: Addresses exist at other protocol layers as well. A computer will often have both an IP (layer 3) and a MAC (layer 2 medium access control such as Ethernet) address, such as my computer s B8-EE-65-F2-27-A8.

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42 Private IP addresses There is a pool of private IP addresses that can be used over and over in separate local networks (like the one controlled by your wireless router). This avoids the unnecessary and wasteful assignment of a permanent IP address to each and every device in your home Ethernet Wireless router: Private address: Regular IP address assigned by ISP: Cable or FiOS Internet For a 16-bit address block, the range of private IP addresses is (RFC 1918)

43 DHCP (Dynamic Host Control Protocol) Dynamic Host Configuration Protocol (DHCP) automatically provides an Internet Protocol (IP) host with its IP address and other related configuration (RFCs 2131 and 2132). The DHCP server maintains a pool of IP addresses and leases an address to a DHCP-enabled client when it starts up on the network. Because the IP addresses are dynamic (leased) rather than static (permanently assigned), addresses no longer in use are automatically returned to the pool for reallocation. Wireless router: Regular IP address leased by ISP: FiOS Verizon DHCP server Internet

44 The DHCP server s database includes: -Valid TCP/IP configuration parameters for all clients on the network. -Valid IP addresses, maintained in a pool for assignment to clients. -Reserved IP addresses associated with particular DHCP clients. This allows consistent assignment of a single IP address to a single DHCP client. -The lease duration. [Some Windows servers have 8 day default.] A DHCP-enabled client, upon accepting a lease offer, receives: -A valid IP address for the subnet to which it is connecting. -Requested DHCP options, such as Router (default gateway), DNS Servers, and DNS Domain Name.

45 IPv6 (Internet Protocol version 6) -Greatly expands the number of available IP address by using a 128 bit address space rather than IPv4 s 32 bit address space. -Implementation has been slow. Most, but not all, DNS root servers can handle IPv6 addresses. [internetsociety.org/ipv6-frequently-askedquestions#one] -Although IPv4 addresses are almost completely depleted, the use of network address translation (NAT), similar to DHCP, can handle some of the increasing need for additional addresses. -The Internet of Things, when socks and light bulbs may have IP addresses, will increase the urgency for adoption of IPv6.

46 0 Bits Version Traffic class Flow label IPv6 datagram format [Different priority classes] Payload length Stream identifier, usually audio or video Next hdr. Source IP address (128 bits) Hop limit In payload, header such as TCP or UDP (Max. remaining hops) Destination IP address (128 bits) 320 Data (usually from a TCP or UDP data unit)

47 TCP and UDP These transport (level 4) protocols provide a transport service for applications, connecting ports (next slide) on the sending and receiving computers. Each of these transport protocols provides a transport data unit which we stuff into IP packets. Transport data unit IP vehicle (packet)

48 Transport protocol provides an end to end transport service. Example: IP packet 25 Destination host application message port 25 TCP processor Destin. port specified: 25 First part of TCP or UDP protocol data unit IP processor

49 TCP (transport control protocol) provides a reliable, but not always fast, connection-oriented transfer service, useful for most computer data. It also does transmission rate control in response to congestion. Information may be slowed, but is almost certain to get through. UDP (user datagram protocol) adds little to IP, providing a best effort, but fast, transfer service, useful for real-time speech, audio and video.

50 Aside: Definitions of connection-oriented and best effort (connectionless) -A connection is an arrangement for an orderly flow of data from a source to a destination. Requires setup/teardown signaling. TCP sets up a reliable connection. It is built on top of the unreliable IP datagram service. -The IP best effort datagram service is connectionless. Each individual packet is on its own, with no effort to coordinate the flow. Packets may travel different routes and arrive out of order. In either case, the transport data units are stuffed into IP packets: IP TCP IP UDP

51 Examples of best effort (connectionless) and connectionoriented data flows Connectionless Datagram: No resource reservation, no path or ordering guarantees (e.g., UDP) Connection-oriented No resource reservation (e.g., TCP) Resource reservation (circuit or virtual circuit) UDP over datagrams C B A Router A A Connection destination TCP over datagrams C B A C B Connectionless destination

52 TCP and UDP Data Unit Formats TCP Bits 16 Code for special handling 32 Source port Destination port Sequence number Ack number Data offset or res.bits Window (receive buffer) size Checksum Pointer to end of urgent data Options Padding UDP Source port Destination port Length [No. of octets in entire data unit] Checksum Computer port identifies a particular application (such as or Web browsing), and possibly also a particular host computer.

53 TCP Connection and Rate Control Handshake protocol establishes a connection. Connection assures that packets are restored to the proper order at receiving end. Packs are retransmitted if incorrectly received (or not received at all). Sliding window flow control throttles back source if there is congestion (evidenced by delivery delays), and lets rate increase if delivery looks good. Start-up at modest rate, increase if network clear. Linear rate of increase, exponential (much faster) rate of decrease when trouble is detected. Packet transmission rate No congestion Congestion time

54 UDP doesn t do any of that fancy stuff Packets may be lost en route, and it is up to the application to fill in the missing data as best it can. The benefit is no retransmission delays. Occasional bits of missing data do not degrade speech, audio and video significantly. That is why UDP is used for VoIP (voice over IP) and media streaming, discussed in the next lecture. UDP over datagrams Router A A Packet A dropped C B A C B C VoIP destination port BVoIP application (copes without A)

55 This concludes lecture 3. See you next week when we focus on Internet applications.